Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as the oxides of nitrogen (NOx), and unburned solids known as particulate matter. Due to increased awareness of the environment, exhaust emission standards have become more stringent and the amount of NOx and particulate matter emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to help ensure compliance with the regulation of NOx, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR). SCR is a process where gaseous or liquid reductant (most commonly urea) is added to the exhaust gas stream of an engine and is adsorbed onto a downstream catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2, both of which are innocuous substances. Although SCR can be an effective way to reduce NOx, urea and/or sulfur deposits can form within the catalyst during the process, thereby decreasing an effectiveness of SCR. One way of removing these deposits includes periodically heating the exhaust passing through the catalyst above a reaction temperature of the deposits.
The particulate matter in an engine's exhaust is commonly collected with a particulate filter. Over time, the particulate matter builds up in the filter and, if left unchecked, the saturated particulate filter could negatively affect performance of the engine. As such, the particulate matter is periodically removed from the filter through a process called regeneration. To regenerate the particulate filter, the exhaust flowing through the filter is heated above a combustion threshold of the particulate matter, thereby burning away the collected matter.
One way to periodically heat an engine's exhaust is disclosed in US Patent Publication No. 2008/1096388 (the '388 publication) of Johnson et al. that published on Aug. 21, 2008. Specifically, the '388 publication discloses using an engine brake on at least one cylinder to increase exhaust temperatures to a level sufficient to achieve the regeneration temperature of a diesel particulate filter. During the engine braking described in the '388 publication, fuel is no longer supplied to the braking cylinder. At the same time, an exhaust valve in the braking cylinder is opened just prior to the piston reaching the top-dead-center position to relieve pressures within the cylinder and thereby increase a load on the engine. The increased engine load caused by the braking cylinder results in extra fueling of non-braking cylinders. This extra fueling increases a temperature of exhaust exiting the non-braking cylinders to the required activation temperature of the particulate trap.